1. Field of the Invention
This invention relates to a polyester fiber having an extremely stable inner structure when subjected to heat. More particularly, this invention relates to a polyester fiber having a high modulus of elasticity and a high resistance to fatigue, and able to usefully serve as a fiber for reinforcing a rubber structure having a greatly improved dimensional stability when subjected to heat.
2. Description of the Related Art
It is known that a polyester fiber, particularly a polyethylene terephthalate fiber, has a high strength and a high initial modulus of elasticity and superior characteristics such as a dimensional stability, durability or the like, and accordingly, this polyester fiber is broadly used as a fiber for reinforcing rubber structures such as a V-belt, a conveyor-belt, a tire or the like. In particularly, the above-mentioned characteristics of the polyester fiber satisfy the requirements for a carcass of a radial tire of an automobile, and accordingly, the use of this polyester fiber in the radial tire of the automobile has increased.
Nevertheless, when analyzing the individual properties of the polyester fiber to be used as the fiber for reinforcing the rubber structure, the dimensional stability against heat of the polyester fiber, relative to a heat shrinkability thereof, is inferior to that of a rayon fiber, and a durability of the polyester fiber is lower than that of a polyamide fiber, and accordingly, there is a need to improve the above-mentioned properties.
In particular, when the dimensional stability against heat of the polyester fiber is made better than that of rayon, it is possible to eliminate a postcure-inflation process used for removing strain in the tire generated during the tire molding process, and accordingly, it is expected that the potential of the polyester fiber will become higher, as a fiber for reinforcing the rubber structure and having a superior cost performance than the rayon fiber and the polyamide fiber.
Japanese Unexamined Patent Publication (Kokai) No. 53-58031, No. 57-154410, No. 57-161119, No. 58-98419 or the like discloses a polyester fiber manufacturing method in which an undrawn yarn having a relatively high orientation, i.e., POY, spun from a polyester resin having a high polymerization degree by spinning under a high stress, is drawn to obtain a polyester fiber having an improved dimensional heat stability and an improved resistance to fatigue.
Nevertheless, although the polyester fiber manufactured by the above POY drawing method has an improved dimensional heat stability and improved resistance to fatigue, compared with those of an conventional polyester fiber, when comparing the rayon fiber, the dimensional heat stability of the obtained polyester fiber is still inferior to that of the rayon fiber, and the other properties of the obtained polyester fiber required as the fiber for reinforcing the rubber structure, i.e., a heat stability under an elevated temperature such as a melting point thereof, a strength, a work loss or the like, are not satisfactorily improved.
Further, Japanese Unexamined Patent publication (Kokai) No. 61-41320, No. 62-69819, No. 63-159518, No. 63-165547 or the like discloses a polyester fiber manufacturing method obtaining an undrawn yarn having a higher orientation, by increasing a stress applied to the yarn at a spinning operation an then drawing the undrawn yarn to obtain a polyester fiber having a dimensional heat stability closer to that of the rayon fiber. Nevertheless, the technique disclosed in the above publication is similar to the technique disclosed in the former publications, i.e., Japanese Unexamined Patent Publication (Kokai) No. 53-58031 or the like, in that a spinning under a high stress is attained from the polyester having a high polymerization by increasing a spinning speed and the obtained undrawn yarn is drawn, and further, in that the dimensional heat stability and the characteristics at the elevated temperature of the obtained polyester fiber are not satisfactory.
As described above, these polyester fiber manufacturing methods are characterized in that the polyester having a high polymerization degree is spun at a high spinning speed, as disclosed in the above-mentioned patent publications, to give the undrawn yarn of multifilament a higher orientation. Nevertheless, when a multifilament of the polyester having a high polymerization degree is spun at such a high speed, a cooling between single filaments constituting the multifilament is insufficient and an air current accompanying the filaments becomes larger, and thus a fusion between the single filaments and a fluctuation of the multifilament are generated. As a result, problems arise such as an increase of yarn breakages and fuzz, and that a uniformity of the thickness of the single filaments becomes very poor. When such an undrawn yarn is used, the drawability also becomes poor, and thus the strength and elongation of the obtained polyester fiber, and the processing ability thereof in a twisting process, adhesive treatment or the like, become poor.
Further, a sufficient orientation of the undrawn yarn cannot be attained due to the lowering of the spinning ability, and as a result, it is impossible to obtain any great improvement in the dimensional stability and the properties of the polyester fiber under an elevated temperature.